1. Field of the Invention
The present invention relates to a method for manufacturing transformer cores wound from a strip of amorphous magnetic alloy.
2. Description of the Related Art
Recently, amorphous magnetic alloys have attracted attention as core materials because of low magnetic loss and methods for manufacturing distribution transformers using a strip of amorphous magnetic alloy have been studied.
Conventionally, cores for distribution transformers are made using silicon iron strips. According to a conventional manufacturing method, laminations each of which is comprised of plural silicon iron strip elements having been cut so as to have a length slightly larger than that of one turn are prepared and bent into a rectangular configuration. Then, plural laminations are packaged into a lamination block by abutting the ends of each of laminations stepwise. Further, plural lamination blocks are built into a core. This type of core is called a core of one turn cut.
Upon manufacturing the core of one turn cut type, a wound silicon iron strip is cut with a length slightly longer than that of one turn while unwinding the wound strip. The cut strip elements are wound successively so as to form a circular core by staggering the joint position thereof to that of the foregoing one. The core thus formed is shaped into a rectangular configuration and, thereafter, annealed. Then, the core is opened once to windings thereinto and, thereafter, the core is close by jointing respective joints to build into a transformer.
An applicability of this manufacturing method for the manufacture of cores using amorphous magnetic strip was studied at first. However, the amorphous magnetic strip is difficult to handle since it has a thickness of about 25 .mu.ms which demands a laborsome and inefficient cutting operation.
In order to solve these problems, there has been proposed a method wherein lamination units are formed by laminating plural strip elements (ten to several tens element and a core is formed using plural lamination units. According to this method, plural lamination units are stacked and the stacked units are wound to form a lamination block by lapping both ends of each of the units shifted with each other in stairstep fashion and plural lamination blocks thus formed are built to a core having a predetermined thickness.
In this conventional method, the strip is wound around a mandrel having a length one the outer periphery thereof longer than that of the inner periphery of the core to be formed (usually longer by several tens mms). Then, the strip wound around the mandrel is cut off at a predetermined position thereof to obtain a lamination unrolled linearly. Then, lamination units are formed by separating plural strip elements so as to have a predetermined thickness, respectively.
In this method, it is necessary to give a sufficient lapping length at each joint in order to obtain a desirable magnetic property of the core. Upon building the transformer using cores, respective joints of the core are opened once after annealing it and the opened core is inserted from a window of the transformer and, thereafter, respective joints are joined again. If the lapping amount of each joint is too short, it becomes difficult to join respective opened joints again. Due to these reasons, it becomes necessary to guarantee the lapping amount of about six times the thickness of the lamination unit at respective joints.
In the core formed according to the lapping joint method, the thickness at each lapped joint becomes larger than those of other portions of the core and the amount of increase of the thickness becomes large in the radially outward direction of the core. Due to this, the length of outer periphery of the core is increased and, on the contrary, the lapping amount at each joint is decreased thereby.
According to the conventional manufacturing method wherein individual cores are formed using plural lamination units each of which is comprised of plural strip elements, the strip elements of the inner lamination block can be lapped a lapping amount slightly smaller than the increase of the inner periphery of the inner lamination relative to that of the core.
However, in this conventional method, it is impossible to guarantee a sufficient lapping amount at outside joints because of the increasing radius at the respective joints. If the lapping amount at the inner joint is set beforehand at a value fairly larger than that necessary for the inner joint, it becomes possible to guarantee the necessary lapping amount at the outer joint. However, this necessitates much more volume of the amorphous magnetic material and is uneconomical.